The present invention relates to a method for controlling implements with a hydraulic circuit employing an accumulator, and more particularly to a method for controlling implements with first and second hydraulic circuits in which a particular condition of an object of control generated by the first hydraulic circuit is detected and the second hydraulic circuit is actuated on the basis of the detection signal.
For instance, in a secondary pressurization casting process of the type that a secondary pressurizing force is applied to molten metal filled within a cavity of a metal mold under pressure, the above-mentioned hydraulic control method is employed.
A pressurizing die casting process in which molten metal is poured into a cavity of a metal mold under high pressure, has been widely employed and practiced as a most suitable process for mass-production in the art of casting of aluminium alloys. This die casting process has a tendency that blow holes are liable to be produced in a thick wall portion of a product, a crystalline structure becomes coarse and degradation of a strength in the thick wall portion is resulted. In order to resolve this problem, a secondary pressurization casting process in which an additional pressurizing force (secondary pressurizing force) is exerted upon molten metal filled within a cavity of a metal mold under pressure before solidification, was proposed (for example, see Japanese Patent Publication No. 48-7570 (1973), Japanese Patent Publication No. 49-36093 (1974), U.S. Pat. No. 3,106,002, U.S. Pat. No. 4,446,907, U.S. Pat. No. 4,497,359 and UK Patent Application GB 2055316A ).
The die casting process has the characteristic nature that since molten metal within a pressurized injection sleeve is injected into a cavity of a metal mold in a jet-like manner through an extremely narrow injection path and then solidified, the time before completion of solidification is very short. Therefore, a latitude in timing for performing secondary pressurization is small, and it is difficult to define the timing for starting the pressurization. If this timing is wrong, not only the effect of pressurization cannot be attained, but also in the case where the secondary pressurization has been carried out after commencement of solidification, sometimes cracks may be produced at the pressurized portion.
In the prior art, various procedures were proposed, in which a position of an injection ram for pressurizing molten metal within an injection sleeve is detected by means of a position sensor or the like and the timing for secondary pressurization is defined by making use of the detection signal as a trigger signal. However, according to these proposed procedures, variation of detection accuracy caused by change of performance of a position sensor, change in a speed of an injection ram, change of a volume of molten metal and the like, is large, and so, it was impossible to practice the secondary pressurization under high reliability.
In addition, another procedure of the type that secondary pressurization is effected after the molten metal injected into a cavity has been pressurized at an increased pressure by jointly employing an injection cylinder and a pressure booster cylinder, in which the pressure in the injection cylinder is detected and the timing of the secondary pressurization is defined by making use of the detection signal as a trigger signal, was also known. However, according to the last-mentioned procedure, since the pressure within the injection cylinder which rises abruptly after completion of filling of molten metal is detected, the detection timing is unstable, and so, the time point for commencing the secondary pressurization could not be defined accurately.